BERLIN—German Chancellor Angela Merkel complained to President Barack Obama on Wednesday after learning that U.S. intelligence may have targeted her cellphone, saying that would be “a serious breach of trust” if confirmed.

For its part, the White House denied that the U.S. is listening in on Merkel’s phone calls now.

“The president assured the chancellor that the United States is not monitoring and will not monitor the communications of the chancellor,” White House spokesman Jay Carney said. “The United States greatly values our close co-operation with Germany on a broad range of shared security challenges.”

However, Carney did not specifically say the U.S. had never monitored or obtained Merkel’s communications.

The German government said it responded after receiving “information that the chancellor’s cellphone may be monitored” by U.S. intelligence. It wouldn’t elaborate, but German news magazine Der Spiegel, which has published material from NSA leaker Edward Snowden, said its research triggered the response.

Merkel spokesman Steffen Seibert said in a statement the chancellor made clear to Obama in a phone call that “she views such practices, if the indications are confirmed . . . as completely unacceptable.”

Merkel said among close partners such as Germany and the U.S., “there must not be such surveillance of a head of government’s communication,” Seibert added. “That would be a serious breach of trust. Such practices must be stopped immediately.”

Carney, the White House spokesman, said the U.S. is examining Germany’s concerns as part of an ongoing review of how the U.S. gathers intelligence.